1. Field of the Invention
The present invention relates to fuel cell assemblies having ion-exchange polymer membranes. More particularly, it relates to the distribution plates for distributing the fluids used in such fuel cell assemblies, that is to say the bipolar plates installed between each of the elementary electrochemical cells and the end plates installed on either side of the stack of the various electrochemical cells.
2. Description of Related Art
The bipolar plates used in fuel cell assemblies fulfill two very different functions. It is known that the fuel cell assembly has to be supplied with fuel gas and with oxidant gas, i.e. with hydrogen and air or pure oxygen respectively, and that the cell must also be cooled, that is to say a coolant such as water must pass through it. One of the functions of the bipolar plates is to allow passage for these various fluids needed for the operation of the fuel cell assembly. Moreover, the bipolar plates also fulfill an electrical function, namely to provide electrical conduction between the anode and the cathode of each of the adjacent electrochemical cells. Now, a fuel cell assembly always consists of a large number of elementary electrochemical cells assembled in series, the elementary electrochemical cells being connected in series, and the nominal voltage of the fuel cell assembly is the sum of the voltages of each elementary electrochemical cell.
These various functions, conveying fluids and conducting electricity, determine the specifications that the materials used for producing these bipolar plates must meet. The materials used must have a very high electrical conductivity. The materials used must also be impermeable to the fluids used and provide very great chemical stability with respect to these fluids.
Furthermore, the bipolar plates must have sufficiently high mechanical properties to allow the juxtaposition of a large number of elementary electrochemical cells and associated bipolar plates and to hold the assembly together by compression between end plates thanks to rods. The bipolar plates must have sufficiently high mechanical properties to withstand this compression. Graphite is commonly used, since this material offers both a high electrical conductivity and chemical inertness to the fluids used. Patent application WO 2005/006472 shows one possible embodiment of such bipolar plates. It may be seen that they consist of the superposition of two relatively rigid graphite sheets with interposition of a foil made of a relatively flexible graphite material so as to accommodate thickness tolerances on the various layers. The graphite sheets include networks of channels needed for distributing the fuel and oxidant gases, that is to say hydrogen and air or pure oxygen, and the channel allowing a coolant such as water to pass through each bipolar plate.
Unfortunately, the rigid components contributing to the construction of the graphite bipolar plates have quite a low impact resistance, in particular during handling when the fuel cell assembly is being assembled. The layer made of the abovementioned flexible graphite material is also most particularly difficult to handle in an industrial situation. All this significantly increases the manufacturing cost of such bipolar plates.
U.S. Pat. No. 6,379,476 proposes to produce bipolar plates made of stainless steel coated on the surface with a passivated film and having carbide inclusions protruding at the surface. According to the applicant of that patent, the proposed product must have a sufficiently low electrical contact resistance in order for bipolar plates to be made therefrom. However, although this solution may have a few advantages over bipolar plates made entirely of graphite, especially as regards mechanical properties, it remains complicated to implement and the electrical resistivity may prove to be too high, especially if the aim is to achieve a very high power density for the fuel cell assembly.
Patent application US 2005/0221158 describes a composite bipolar plate formed from a steel support having a conductive anti-corrosion coating, the channels being formed in a graphite layer pierced in places in order to define the channels. This solution therefore has the same drawbacks as those mentioned in the comment regarding patent application WO 2005/006472.
Other patent applications propose to produce bipolar plates made of non-metallic materials, for example a plastic, because many of these materials are very insensitive to chemical attack due to the gases and to the coolant that are used. One example that may be mentioned is patent application WO 2006/100029.
The use of metal plates as bipolar plates offers many advantages over graphite plates. The main advantage to be mentioned is the superior mechanical strength of metal, which makes it possible to reduce the thicknesses of the plates and to avoid plate cracking problems. The metal combined with a particular surface treatment is a good solution for the cathode side (for chemical reduction reactions). Its corrosion resistance and its conductivity are good. However, on the anode side (for chemical oxidation reactions), even with a good surface treatment, corrosion appears after only a few hundred hours of operation.
The objective of the present invention is to propose an arrangement for a gas distribution plate for an electrochemical cell of a fuel cell assembly that avoids the drawbacks mentioned above, while also being as robust as possible and quite easy to manufacture.